High temperature electrolyzer (hte) including a plurality of cells, having improved operation in the event of breakage of at least one cell and during ageing

ABSTRACT

A process of electrolyzing water at high temperatures implemented by a cell stack reactor, including: a) simultaneously circulating water vapour at each cathode and at each anode as a leaching gas, temperatures of the water vapour at an inlet of each anode and each cathode being lower than high temperatures at which electrolysis is carried out and the water vapour circulating at the anode being at an overpressure with respect to the cathode; b) upon starting the electrolysis, supplying electrical power having a substantially constant electrical voltage across terminals of the stack and maintaining same. In event of breakage of one or more cells, complete destruction of the stack is avoided and high production efficiency is maintained, and efficiency is maintained during ageing.

TECHNICAL FIELD

The invention relates to a process of electrolysing water at hightemperatures (HTE), also known as high temperature vapour electrolysis(HTVE), with a view to producing hydrogen.

It also relates to a reactor for implementing said process.

More particularly, it relates to an improvement in the reliability ofoperation and the efficiency of high temperature electrolysers (HTE), inthe event of potential breakages of one or more cells.

In addition, it relates to an improvement in the efficiency of said HTEelectrolysers, during ageing or in other words after a considerable timeof use.

PRIOR ART

An electrolyser comprises a plurality of elementary cells formed of acathode and an anode separated by an electrolyte, the elementary cellsbeing electrically connected in series by means of interconnectingplates interposed, in general, between an anode of an elementary celland a cathode of the following elementary cell. An anode-anodeconnection followed by a cathode-cathode connection is also possible.The interconnecting plates are electronic conductive components formedof a metal plate. Said plates moreover ensure the separation between thecathodic fluid circulating at an elementary cell from the anodic fluidcirculating in a following elementary cell.

The anode and the cathode are made of porous material into which thegases can flow.

In the case of the high temperature electrolysis of water to producehydrogen, the water vapour circulates at the cathode where hydrogen isgenerated in gaseous form, and a leaching gas can circulate at the anodeand thereby participate in the evacuation of the oxygen generated ingaseous form at the anode. Most high temperature electrolysers use airas leaching gas at the anode.

At present, the construction of a factory for producing hydrogen from alarge number of high temperature electrolysers is being envisaged.

The current dimensioning of such a factory implies in fact thesimultaneous operation of a large number, typically several million, ofelectrolysis cells. These cells are on the one hand fragile and may thusbreak at any moment and, on the other hand, they age and thus produceless hydrogen locally. These two phenomena run counter to the industrialrequirement of large volume hydrogen production. In fact, it isnecessary in this context to have a constant production in quantity andover time and it must do so with great reliability.

In other words, the conception of a factory implies firstly that it isnecessary to envisage the breakage of one or more cells, which leads toeither the stoppage of the electrolyser concerned, or its operation indegraded mode.

Until now, it has been envisaged to compensate for the loss ofproduction stemming from this (these) breakage(s) either by starting upa new high temperature electrolyser or by increasing the power injectedinto each electrolyser not concerned by the breakage(s), in other wordswith all of their cells non-broken.

The drawbacks of these solutions are that this requires an activemanagement with the use of costly electrical cabinets and that, aboveall, the energy efficiency of the HTE electrolyser concerned by thebreakage(s) is reduced.

Secondly, as stated previously, the design of a factory implies that itis necessary to take into account the ageing of all the cells, in otherwords that, in the same conditions (temperature, pressure, current) asthe initial conditions during the start of the electrolysis, theirreactive performances drop.

An aim of the invention is to propose a solution that makes it possiblefor a hydrogen production factory comprising a large number of hightemperature electrolysers to have high reliability and to conserve aconstant production efficiency without the drawbacks of the solutions ofthe prior art.

An aim of the invention is thus to propose a solution that makes itpossible, at lower cost, not to suffer losses of efficiency of a hightemperature electrolyser (HTE) due to potential breakages of cells andmoreover to the ageing thereof.

DESCRIPTION OF THE INVENTION

To do this, the invention relates to a process of electrolyzing water athigh temperatures implemented by an electrochemical reactor comprising astack of N elementary electrochemical cells each formed of a cathode, ananode and an electrolyte inserted between the cathode and the anode, atleast one interconnecting plate being arranged between two adjacentelementary cells and in electrical contact with an electrode of one ofthe two elementary cells and an electrode of the other of the twoelementary cells, in which at least water vapour is made to circulate incontact with the cathode and a leaching gas is made to circulate incontact with the anode to evacuate the oxygen produced, characterised inthat the following steps are carried out:

a/ simultaneously circulating the water vapour containing at the most 1%of hydrogen at each cathode and at each anode as a leaching gas, thetemperatures of the water vapour at the inlet of each anode and eachcathode being lower than the high temperatures at which the electrolysisis carried out and the water vapour circulating at the anode being at anoverpressure compared to the cathode,

b/ imposing upon starting the electrolysis and maintaining asubstantially constant level of electrical voltage across the terminalsof the stack of electrolysis cells.

The expression “the temperatures of the water vapour at the inlet ofeach anode and each cathode being lower than the high temperatures atwhich the electrolysis is carried out” is taken to mean, within thescope of the invention, that a slightly exothermic operation of theelectrolyser is sought to target a stable, in other words auto-thermal,operation of the assembly constituted of the electrolyser(electrochemical reactor) and the associated heat exchange system.Upstream and downstream of an electrolyser is installed a heat exchangersystem, the function of which is to use the heat of the outgoing gasesto heat the incoming gases (here the non-hydrogenated water vapour).They ensure the thermal stability of the assembly, thus a slightlyexothermic operation of the electrolyser is aimed at here.

Thus, the water vapour containing at the most 1% of hydrogen enters intothe electrolyser at temperatures below (heat at low temperatures) thehigh operating temperatures and is reheated thanks to the energydissipated by Joule effect (thus of electrical origin) in the core ofthe electrolyser, in other words within each cell.

The cell breakage configurations envisaged within the scope of theinvention are those that do not lead to an interruption of theelectrical connection at the cell but only create a hydraulic“short-circuit” between anode and cathode. The inventors have thus beenable to observe that these breakage configurations were those typicallyobserved in practice, in other words with the architectures anddimensioning of electrolysers already known to date. It goes withoutsaying that those skilled in the art will take care, within the scope ofthe invention, to ensure that the architecture and the dimensioning ofan electrolyser do not lead to breakage of electrical connection at eachcell.

Thus, the solution according to the invention makes it possible tooperate a high temperature electrolyser without, or with little,efficiency losses due to potential breakages of one or more cells, andto do so without it being necessary to involve active compensationmanagement.

In other words, the electrolyser reacts itself and in a reliable mannerto the phenomena of breakage of cells by reducing any risk of seriousdamage.

The solution according to the invention thus consists in a combinationof means for carrying out respectively:

an auto-thermal operation of the assembly constituted of theelectrolyser and the associated heat exchanger(s),

over-pressurising water vapour at the anode,

a constant electrical voltage across the terminals of the stack.

Thus, in the event of breakage of a cell, thanks to the overpressure ofnon-hydrogenated water vapour circulating at the anode, the leak causedby this breakage is directed from the anode to the cathode. In otherwords, a flow of water vapour more or less loaded with oxygen arrives atthe cathode. The oxygen present then reacts with the hydrogen produced,which again generates additional water vapour with a release of heat.The presence of the flow of water vapour from the anode moderates therise in temperature. Nevertheless, this moderate rise in temperatureimproves the electrical conductivity at the part of the cathodedownstream of the breakage, which consequently reduces the production ofheat by Joule effect of the initial operation, in other words before thebreakage.

All of the additional flows of water vapour at the cathode due on theone hand to the recombination of oxygen coming from the anode via thebroken area with the hydrogen already present at the cathode and at theleak (water vapour already present at the anode), leads to aredistribution of the current in the circulation channel in contact withthe cathode (Nernst potential, Butler-Volmer equation).

The following phenomena occur:

At the Broken Elementary Cell:

The electrical voltage across the terminals of the broken elementarycell drops: in fact, the quantity of water vapour is greater, the brokenelementary cell is hotter. The electrical voltage across the terminalsof the cell being lower, the operation of the broken elementary cell maybe considered exothermic, in other words that the local electrolysisdownstream as upstream of the breakage consumes part of the excess heat.

The conditions of gas, temperature, downstream of the breakage favour anelectrolysis downstream rather than upstream of the breakage. In fact,as mentioned previously, these conditions lead to an electricalconductivity in the downstream part of the cathode. Yet, the totalcurrent per cell is imposed by the constant voltage across the terminalsof the stack of cells. Thus, due to this greater electrical conductivitydownstream of the breakage and the total current imposed at the brokenelementary cell, there are less electrochemical reactions upstream ofthe breakage.

Considering a stack of a number of N+1 cells, with the number N veryhigh for example N−1000.

In the absence of breakage, the relations linking the voltage across theterminals of a cell u^(cel), across the terminals of the stack of N+1cells with the current may be written as follows:

U ₀ =N u ₀ +u ^(cel) ₀ u ^(cel) ₀ =u ₀ and I ₀ =i ₀,

in which U₀ is the voltage maintained constant across the terminals ofthe stack according to the invention. In these equations and in thefollowing equations, by convention, upper case letters are used for whattakes place at the terminals of the stack, and lower case letters areused for what takes place on a cell concerned by the breakage.

Following the breakage of a cell, the voltage across the terminals ofthe cell concerned is written:

ucel=ucel₀−ε

From which:

N u ₀ =N u−ε

u being the voltage on the other non-broken cells.

The preceding equation can also be written:

u=u ₀ +ε/N

Considering the value R of apparent resistance of a cell, the followingrelation is obtained:

u₀=R i₀

and

u=R i

from which the value of the current i in each of the other non-brokencells:

i=i ₀ +ε/NR.

Thus, the inventor has arrived at the conclusion that the variationsinduced by a breakage of a cell on the other unbroken cells are smallerthe higher the value of N. Yet, in practice, in the stacks of cellsenvisaged within the scope of the invention, it is the case.

There are thus fewer losses by recombination of products of the upstreamelectrochemistry (hydrogen produced upstream of the breakage and oxygencoming from the anode via the leakage at the broken area). Theelectrolysis occurring locally downstream of the breakage participatesin the overall production of hydrogen by the stack of cells.

It may thus be considered that the complete electrolyser has in a wayitself reacted to reduce the risks of serious damage.

At the other Non-Broken Elementary Cells:

Due to the fact that the electrical voltage on either side of the brokencell has dropped, and that the complete stack of cells is under aconstant imposed electrical voltage, the other non-broken elementarycells are under a slightly increased elementary voltage. The elementarycurrent thus consequently increases slightly, which ensures an excess ofoverall production of hydrogen by the stack of cells and compensates theshortfall due to the breakage.

The overpressure of the water vapour containing at the most 1% ofhydrogen at the anode compared to that at the cathode may be comprisedbetween 5 and 100 mbars, preferably 30 mbars.

The number N of elementary cells and the level of voltage imposed andmaintained constant are such that the unitary voltage level across theterminals of each elementary cell is of the order of 1.3 volts. Thisvalue corresponds to the voltage that enables the assembly constitutedof the electrolyser associated with the heat system to have a stableoperation, in other words auto-thermal operation, with, if necessary,some heat losses. It goes without saying that this value is determinedfor the water vapour containing at the most 1% of hydrogen.

The initial conversion rate into hydrogen is preferably of the order of100%.

According to an advantageous embodiment, the flow rate of water vapourat each cathode is increased when the conversion rate into hydrogeninitially determined at the outlet of each cathode drops. A givenproduction rate that does not drop is thereby guaranteed.

The expression “conversion rate into hydrogen at the outlet of thecathode” is taken to mean the proportion of water vapour at the inlet ofthe cathode, which is transformed by electrolysis into hydrogen at theoutlet of the cathode. Thus, if at the inlet of the cathode,non-hydrogenated water vapour is made to circulate, and that theconversion rate initially determined is 100%, one collects, apart fromany breakage, uniquely the hydrogen at the outlet of the cathode. Thoseskilled in the art will take care to determine the necessary surface ofeach elementary cell and the initial flow rate of water vapour at eachcathode to arrive at the desired initial conversion rate. They will thentake care through design to over-dimension the necessary cell surface.

In this way, it is ensured that the ageing of the cells does notadversely affect the hydrogen production efficiency. In fact, thereserve of cell surface by over-dimensioning thereof that does not servethe electrolysis before ageing and which is situated downstream, come tobe used when the cell ages. Thus, the conversion rate of a cell and theefficiency thereof remain correct.

Obviously, if the initial conversion rate determined is of the order of100%, those skilled in the art will take care to install a condensationstage to condense the water vapour not converted during ageing.

As of considerable ageing, when all of the surface of the cell isalready used, the conversion rate is going to decrease and thus the flowrate of hydrogen also. This behaviour is the normal behaviour ofelectrolysers.

If it is wished to maintain this hydrogen flow rate and conserve goodthermodynamic efficiency, it is necessary to increase the flow rate ofwater vapour to the detriment of the utilisation rates.

The process can operate at temperatures of at least 450° C., typicallycomprised between 700° C. and 1000° C.

The invention also relates to a device for electrolysing water at hightemperatures, comprising an electrical voltage source and a reactorcomprising a stack of elementary electrochemical cells each formed of acathode, an anode and an electrolyte inserted between the cathode andthe anode, at least one interconnecting plate being arranged between twoadjacent elementary cells and in electrical contact with an electrode ofone of the two elementary cells and an electrode of the other of the twoelementary cells, the interconnecting plate comprising at least onecathodic compartment and at least one anodic compartment for thecirculation of gases respectively at the cathode and the anode.

According to the invention, one of the ends of the cathodic compartmentsis connected to a supply adapted to deliver water vapour containing atthe most 1% of hydrogen and one of the ends of the anodic compartmentsis also connected to a supply adapted to deliver water vapour containingat the most 1% of hydrogen at an overpressure compared to those of thecathode, the supplies being adapted to deliver the water vapour attemperatures below those at which the electrolysis is carried out, thedevice comprises means connected to the electrical voltage source todeliver a substantially constant voltage U₀ across the terminals of twointerconnecting plates of the stack the furthest away from each other.

Finally, the invention relates to a hydrogen production assemblycomprising a plurality of devices such as that described above.

BRIEF DESCRIPTION OF DRAWINGS

Other advantages and characteristics will become clearer on reading thedetailed description given for illustration purposes and non-limiting,and by referring to the following drawings, among which:

FIG. 1 is a side view of an embodiment of a reactor for electrolysis athigh temperatures according to the present invention,

FIG. 1A is a sectional view of the reactor of FIG. 1 along the plane A-Ain electrolysis operation without breakage of cells,

FIG. 1B is a sectional view of the reactor of FIG. 1 along the plane B-Balso in electrolysis operation without breakage of cells,

FIG. 2 is a view analogous to FIG. 1B but schematically showing abreakage of a cell,

FIGS. 3A, 3B, and 3C show schematically a distribution of the currentalong a channel in an electrolysis reactor according to the inventionrespectively in operation without breakage of electrolysis cells, inoperation with a breakage localised in a first cell area and, inoperation with a breakage localised in a second cell area distinct fromthe first area,

FIG. 4 schematically shows the evolution of the conversion rate intohydrogen along a cell according to the invention and not havingundergone ageing.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

The invention is described in relation to a type of high temperaturewater electrolyser architecture for producing hydrogen. It goes withoutsaying that the invention can apply to other architectures. The hightemperatures at which the represented electrolyser operates are at leastequal to 450° C., typically comprised between 700° C. and 1000° C.

It is pointed out that the terms “upstream” and “downstream” are usedwith reference to the direction of circulation of the water vapour andthe hydrogen produced at the cathode.

It is pointed out that the representations of the different componentsare not shown to scale.

Finally, it is pointed out that the representation of the distributionof the current is in the form of straight line segments in FIGS. 3A to3C for simplification: it goes without saying that in reality, thesecurrent distributions are decreasing curve portions.

In FIG. 1 is represented an HTE electrolyser according to the presentinvention comprising a plurality of stacked elementary cells C1, C2 . .. .

Each elementary cell comprises an electrolyte arranged between a cathodeand an anode. In the remainder of the description, the cells C1 and C2and their interface will be described in detail.

The cell C1 comprises a cathode 2.1 and an anode 4.1 between which isarranged an electrolyte 6.1, generally of 100 μm thickness for the cellsknown as electrolyte support and of several μm thickness for the cellsknown as cathode support.

The cell C2 comprises a cathode 2.2 and an anode 4.2 between which isarranged an electrolyte 6.2.

The cathodes 2.1, 2.2 and the anodes 4.1, 4.2 are made of porousmaterial and have for example a thickness of 40 μm for the cells knownas electrolyte support and a thickness of the order of 500 μm for thecathode of the cells known as cathode support and 40 μm for the anode.

The anode 4.1 of the cell C1 is electrically connected to the cathode2.2 of the cell C2 by an interconnecting plate 8 coming into contactwith the anode 4.1 and the cathode 2.2. Furthermore, it enables theelectrical power supply of the anode 4.1 and the cathode 2.2.

An interconnecting plate 8 is interposed between two elementary cellsC1, C2.

In the example represented, it is interposed between an anode of anelementary cell and the cathode of the adjacent cell. But, it could beprovided that it is interposed between two anodes or two cathodes.

The interconnecting plate 8 defines with the adjacent anode and thecathode channels for the circulation of fluids. More precisely, theydefine anodic compartments 9 dedicated to the circulation of gases atthe anode 4 and cathodic compartments 11 dedicated to the circulation ofgases at the cathode 2.

In the example represented, an anodic compartment 9 is separated from acathodic compartment by a wall 9.11. In the example represented, theinterconnecting plate 8 comprises in addition at least one conduit 10delimiting, with the wall 9.11, the anodic compartments 9 and thecathodic compartments 11.

In the example represented, the interconnecting plate 8 comprises aplurality of conduits 10 and a plurality of anodic 9 and cathodic 11compartments. In an advantageous manner, the conduit 10 and thecompartments have hexagonal, honeycomb sections which makes it possibleto increase the density of compartments 9, 11 and conduits 10. Othersections can also be suitable for the sections of the compartments.

As represented in FIG. 1A, water vapour containing at the most 1% ofhydrogen is made to circulate at each cathode 2.1, 2.2 and at each anode4.1, 4.2 as a leaching gas. The arrows 12 and 13 of FIG. 1A thus clearlyrepresent the simultaneous path in the anodic 9 and cathodic 11compartments. It goes without saying that within the scope of theinvention the flow symbolised may just as easily be in the otherdirection (arrows 12 and 13 in the opposite direction). As representedin FIG. 1B, the architecture of the electrolyser makes it possible inaddition to connect the first end 10.1 of the conduit 10 to a supply ofwater vapour containing at the most 1% of hydrogen via another conduitand to connect the second end 10.2 of the conduit 10 to the cathodiccompartment 11. The arrow 14 thus symbolises the return flow of thewater vapour from its flow in the conduit 10 (arrow 16) to the cathodiccompartment 11. It is pointed out here that the initial circulation inthe conduit 10 of the water vapour makes it possible to homogenise thetemperatures and thus avoid heat gradients capable of damaging thecells.

According to the invention, upon starting a water electrolysis cycle,care is firstly taken initially to ensure a slightly exothermicoperation of the electrolyser at high temperatures: thus the watervapour circulating at the inlet 11.1 of each cathodic compartment 11 isat lower operating temperatures, in other words those at which theelectrolysis of the water along each cathodic compartment is carried outby heating the vapour using the energy dissipated by Joule effect.

Typically, the temperatures at the inlet of the cathodic compartment11.1 are of the order of 800° C. for temperatures of operation (duringthe electrolysis along the cathodic compartment 11) which can reach 820°C.

According to the invention, the water vapour circulating in the channelor anodic compartment 9 is also over-pressurised (arrows 12) compared tothat circulating in the channel or cathodic compartment 11 (arrows 13).Typically, the overpressure is comprised between 15 and 100 mbars,preferably of the order of 30 mbars.

Finally, the electrical voltage U₀ at the terminals of the stack ofcells delivered by the power supply source 15 is maintainedsubstantially constant. An electrical voltage U₀ is advantageouslychosen such that for a stack of N electrolysis cells C1, C2 . . . Cn,the average unitary voltage across the terminals of each cell

${U_{1} = \frac{U_{0}}{N}},$

i.e. substantially equal to 1.3 Volts.

In FIG. 2 is represented a situation of breakage of cell C1 typicallyobserved in already tested electrolysers: the electrolyte 6.1 is brokenbut the electrical connection is still ensured by the interconnectingplate 8.

Due to the overpressure of the water vapour in the anodic compartment 9,a hydraulic short-circuit is in a way created and the water vapourloaded with oxygen already collected flows via the broken part 17 of theanodic compartment 9 to the cathodic compartment (arrow 13.1). The partrepresented broken 17 is voluntarily exaggerated in FIG. 2 and canconsist in reality in a fissure sufficient to allow gases to pass. Theoxygen having passed through the broken area 17 recombines with thehydrogen already present upstream in the cathodic compartment 11 to formwater with a release of heat.

All of the additional flows of water vapour at the cathode 2.1 due onthe one hand to the recombination of the oxygen coming from the anode4.1 via the broken area 17 with the hydrogen already present at thecathode and to the leak (water vapour already present at the anode),leads to a redistribution of the current in the circulation compartment11 in contact with the cathode. Different models, such as the Nernstpotential and the Butler-Volmer law, exist to take this redistributionof the current into account.

In FIG. 3A is represented the distribution line of the current along acathode 2 of an electrolysis cell according to the invention not havingundergone breakage: the surface of the hatched area represents the totalcurrent flowing through the elementary cell. This total current servesintegrally for the local electrolysis at the cell cathode 2.

In FIG. 3B, 3C is represented respectively the segments of distributionline of the current along this same cathode but in a breakage situation,the localisation of the broken area 17 in FIG. 3B being distinct fromthat of FIG. 3C. The surface of the hatched areas here also representsthe total current still applied to the elementary cell. But, here thecurrent represented by the hatched area in solid lines, in other wordcorresponding to the part of the cell downstream of the breakage 17,contributes mainly to the electrolysis at the cell. In fact, the currentrepresented by the hatched area in broken lines, in other words upstreamof the breakage 17, contributes to a minor extent to the electrolysis.

In fact, at the broken elementary cell, the electrical voltage acrossthe terminals of the broken elementary cell drops. The electricalvoltage across the terminals of the cell being lower, the operation ofthe broken elementary cell may be considered endothermic, in other wordsthat local electrolysis downstream as upstream of the breakage consumespart of the excess heat.

The conditions of gas, temperature, downstream of the breakage favour anelectrolysis downstream rather than upstream of the breakage. In fact,as mentioned beforehand, these conditions lead to an electricalconductivity in the part of the cathode downstream of the breakage 17.Yet, the total current per cell is imposed by the constant voltageacross the terminals of the stack of cells. Thus, due to this greaterelectrical conductivity downstream of the breakage 17 and the totalcurrent imposed at the broken elementary cell, there are lesselectrochemical reactions upstream of the breakage 17.

There are thus fewer losses through recombination of products of theupstream electrochemistry (hydrogen produced upstream of the breakage 17and oxygen coming from the anode via the leak 13.1 at the broken area17).

Thus, despite the breakage 17 of a cell, the electrolysis taking placelocally downstream thereof participates in the overall production ofhydrogen by the stack of cells.

The different breakage situations of FIG. 3B and 3C are distinguished bythe fact that, in the configuration of FIG. 3C, the current is not zeroat the outlet of the cathodic compartment 11.2: it may thus beconsidered that, in comparison to the configuration of FIG. 3B, thelocal production of hydrogen is less.

It may thus also be deduced from these examples that the lower theinitial conversion rate (apart from any breakage), the less efficientthe auto-regulation targeted by the invention. It is thus necessary totarget a conversion rate as high as possible, at the best of the orderof 100%.

Different experiments have made it possible to validate the solutionaccording to the invention, namely an overpressure of water vapour atthe anode compared to at the cathode combined with an auto-thermaloperation and a constant electrical voltage across the terminals of thestack of cells. Thus, overall the inventors think that such a solutionmakes it possible not to affect the overall production of hydrogen of aseries of high temperature electrolysers, even in the event of breakageof one or more electrolysis cells.

In FIG. 4 is represented the evolution of the conversion rate ofhydrogen a which takes place through the electrolysis reaction along achannel (cathodic compartment 11) circulating along a cathode 2.i of anelectrolysis cell Ci. As in all the electrolysers, this conversion ratea increases as the gases progress. In the optimal conditions of theinvention, one initially targets, in other words at the design of theelectrolyser according to the invention, a conversion rate a of theorder of 0 at the inlet 11.1 of the compartment, corresponding to anon-hydrogenated water vapour, and of the order of 100% at the outlet ofthe cathodic compartment 11.2, corresponding uniquely to hydrogen.

The inventors started from the principle according to which thisconversion rate α necessarily drops due to the phenomenon of ageing ofthe electrolysis cells in an electrolyser according to the prior art.

They then reached the conclusion that over-dimensioning the cells, inother words providing for an additional surface for the electrolysis,compared to the initially expected electrolysis reaction could againlead to increasing this conversion rate or in other words the hydrogenproduction efficiency. In fact, by combining an increase in theavailable electrolysis surface with an increase in the flow rate ofwater vapour at the inlet if necessary, one shifts in a way moredownstream of the cell the electrolysis during ageing. Thus, thedecreasing conversion rate of hydrogen during ageing is again increasedby an electrolysis shifted downstream in the cell.

The inventors thus think that with an initially targeted conversion rateα of 100% at the cell outlet, an increase of 10 to 20% of the availablesurface of the cell and an increase in the flow rate of water vapour ofthis same order of magnitude if necessary, it is possible despite ageingto conserve a conversion rate of the order of 100%, the reduction of therate then taking place much later.

This may be envisaged even more so given that presently the design ofHTE electrolyser production factories necessarily provides for the useof condensers that could condense the low percentage of vapour notconverted into hydrogen and that, in the near future, an industrialobjective is to produce ceramic electrolytes of dimensions greater than200*200 mm.

The advantages of the solution according to the invention are numerous:

it is simple to implement, reliable and consists in a way in a passiveand instantaneous reactivity of the stack of electrolysis cells, whichdoes not require restrictive measures as in the prior art,

compared to the solutions of the prior art, which imply the use of agreater number of electrolysers in the event of breakage of one or morecells, the process according to the invention is less costly; at themost it is necessary to over-dimension the electrolysis cells comparedto the targeted overall hydrogen production efficiency,

imposing and maintaining a substantially constant electrical voltageacross the terminals of an electrolyser at high temperatures accordingto the invention is simpler than managing a current as in the prior art,

the conditions of auto-thermal operation of electrolyser(s) according tothe invention imply a high production efficiency,

the regulation according to the invention is only carried out within theelectrolyser and it naturally locally adjusts itself, only the increasein the flow rate of non-hydrogenated water vapour needs to be adjustedby the user of the electrolyser depending on his needs (targetedconversion rate).

1-8. (canceled)
 9. A process of electrolyzing water at high temperaturesimplemented by an electrochemical reactor including a stack ofelementary electrochemical cells each formed of a cathode, an anode, andan electrolyte inserted between the cathode and the anode, at least oneinterconnecting plate being arranged between two adjacent elementarycells and in electrical contact with an electrode of one of the twoelementary cells and an electrode of the other of the two elementarycells, in which at least the water vapour is made to circulate incontact with the cathode and a leaching gas is made to circulate incontact with the anode to evacuate oxygen produced, the methodcomprising: a) simultaneously circulating the water vapour containing atmost 1% of hydrogen at each cathode and at each anode as a leaching gas,temperatures of the water vapour at an inlet of each anode and eachcathode being lower than high temperatures at which electrolysis iscarried out, and the water vapour circulating at the anode being at anoverpressure with respect to the cathode; and b) imposing upon startingthe electrolysis and maintaining a substantially constant level ofelectrical voltage across terminals of the stack of electrolysis cells.10. A process of electrolyzing water according to claim 9, wherein theoverpressure of the water vapour containing at the most 1% of hydrogenat the anode compared to that at the cathode is between 5 and 100 mbars,or is 30 mbars.
 11. A process of electrolyzing water according to claim9, wherein a number of elementary cells and a level of voltage imposedand maintained constant are such that unitary voltage across theterminals of each elementary cell is of an order of 1.3 volts.
 12. Aprocess of electrolyzing water according to claim 9, wherein an initialconversion rate into hydrogen is of an order of 100%.
 13. A process ofelectrolyzing water according to claim 9, wherein a flow rate of watervapour at each cathode is increased, when a conversion rate intohydrogen initially determined at an outlet of each cathode drops.
 14. Aprocess of electrolyzing water at high temperature according to claim 9,at temperatures of at least 450° C., or between 700° C. and 1000° C. 15.A device for electrolyzing water at high temperatures, comprising: anelectrical voltage source; a reactor comprising a stack of elementaryelectrochemical cells each formed of a cathode, an anode, and anelectrolyte inserted between the cathode and the anode; at least oneinterconnecting plate being arranged between two adjacent elementarycells and in electrical contact with an electrode of one of the twoelementary cells and an electrode of the other of the two elementarycells, the interconnecting plate comprising at least one cathodiccompartment and at least one anodic compartment for circulation of gasesrespectively at the cathode and at the anode; wherein one of ends of thecathodic compartments is connected to a supply configured to deliverwater vapour containing at most 1% of hydrogen and one of ends of theanodic compartments is also connected to a supply configured to deliverwater vapour containing at most 1% of hydrogen at an overpressurecompared to those of the cathode, the supplies are configured to deliverthe water vapour at temperatures below those at which the electrolysisis carried out; and further comprising means connected to the electricalvoltage source to deliver a substantially constant voltage acrossterminals of the two interconnecting plates of the stack furthest awayfrom each other.
 16. An assembly for producing hydrogen comprising aplurality of devices according to claim 15.